Seismic Enhancement of Masonry‐Infilled Substandard Reinforced Concrete Frames Using Lightweight Steel Exoskeleton

ABSTRACT A significant portion of existing reinforced concrete (RC) structures in seismically active regions was constructed prior to the adoption of modern seismic design standards, leaving them highly susceptible to earthquake‐induced damage. The vulnerabilities of these structures, often exacerbated by material degradation, have been starkly revealed in recent seismic events. This study addresses the urgent need for effective retrofitting solutions by evaluating the seismic performance of deficient masonry‐infilled RC frames retrofitted with a novel lightweight steel exoskeleton system—Resisto 5.9 Tube—designed to enhance structural resilience. Three full‐scale RC frame specimens, replicating typical deficiencies of older construction practices, were subjected to quasi‐static cyclic loading up to near‐collapse conditions, with interstory drifts ranging from 0.05% to 2.50%. The test series included: a bare frame (BF), an infilled frame with unreinforced hollow clay masonry units (IF), and a retrofitted infilled frame (RIF) incorporating the steel exoskeleton. Results reveal that masonry infill substantially increases lateral load capacity—by factors of 2.42 (IF) and 3.59 (RIF) compared to BF. However, IF exhibited a brittle failure mode, with significantly reduced displacement capacity. In contrast, the exoskeleton‐enhanced RIF demonstrated a 147% increase in load capacity relative to IF, extended peak force occurrence to 0.70% drift, and achieved improved cyclic stability. While initial stiffness remained comparable between IF and RIF (within 4% difference), energy dissipation in RIF at 1.50% drift was threefold that of IF. Further, the exoskeleton system markedly improved the performance of the infill wall, extending the defined limit and damage states at ultimate by up to 150% and 344%, respectively. These enhancements facilitated sustained infill–frame interaction under large drifts, a behavior often neglected in conventional seismic design. The findings position the Resisto 5.9 Tube as a cost‐effective and scalable retrofitting solution, offering a paradigm shift in how infill contributions are considered in seismic response assessments. This work establishes a foundation for advanced analytical modeling and practical implementation in earthquake‐prone regions.